Mould making for beginners: alginate, plaster of Paris, and liquid latex
Mould making is one of those craft skills that sits just outside most people's awareness — not because it is particularly difficult, but because the materials involved are less familiar than paint or clay. Once you understand what alginate, plaster of Paris, and liquid latex each do, and how they relate to each other in the mould making process, the whole subject becomes much more accessible. And the things you can make — casts of hands and faces, reproductions of objects, flexible moulds for repeated casting — are genuinely remarkable for a modest investment in materials.
We stock everything you need to get started at Craft and Canvas: Alginate 500g at £19.99, Moulding Powder Plaster of Paris at £9.25, Gedeo Liquid Latex at £10.50, and Seawhite Liquid Latex 1 litre at £27.99. This guide covers how each material works, what the different combinations produce, and how to approach your first mould making project.
The basics: mould and cast
Two words sit at the heart of this subject and are worth defining clearly before going further.
A mould is the negative impression of an object — a shaped cavity or flexible skin that captures the form of whatever was used to make it. The mould is not the final product; it is the tool used to create the final product.
A cast is what you make using the mould — the positive reproduction of the original object, produced by filling or pressing a castable material into the mould and allowing it to set. The cast is the finished piece.
Understanding this relationship — mould captures the form, cast reproduces it — is the foundation of everything that follows. Different combinations of mould making materials and casting materials produce different results, but the principle is always the same.
Alginate: the safest and most versatile mould making material for beginners
Alginate is a powder derived from seaweed that, when mixed with water, produces a gel that sets in minutes and captures surface detail with extraordinary precision. It is completely skin-safe — used extensively in dentistry for taking dental impressions — which makes it the material of choice for life casting: making moulds directly from the human body.
We stock Creative House Alginate at £19.99 per 500g at Craft and Canvas.
How alginate works
Mix the alginate powder with water according to the instructions — typically a ratio of approximately 1 part alginate to 1.5 parts water by volume. The mixture starts as a fluid, pourable liquid and sets to a firm, rubbery gel within a few minutes. The set time varies between alginate products and can also be affected by water temperature — cooler water slows the set and gives more working time, warmer water accelerates it.
Once set, the alginate gel is peeled or demoulded from the original object, leaving a flexible negative impression that captures every detail of the surface — fingerprints, skin texture, the finest relief on an object. This negative is then filled with the casting material of your choice — typically plaster of Paris — to produce a positive reproduction.
The key limitation of alginate
Alginate moulds are single-use. The gel begins to dry and distort once removed from the original object, and should be cast into promptly — ideally on the same day, within a few hours of making the mould. Alginate is not suitable for repeat casting or for moulds you want to keep and reuse. For repeat casting, liquid latex is the appropriate choice — more on that below.
Alginate is also sensitive to hard water in some situations — if your tap water is very hard, using bottled or filtered water gives more reliable results.
What you can do with alginate
Life casting is the most distinctive and impressive application. A cast of a hand, a foot, a baby's hand, or a small face made using alginate and plaster of Paris is one of the most personal and meaningful things you can produce in craft — a precise three-dimensional record of a form at a specific moment in time. The process is straightforward enough that it is achievable at home with basic equipment.
Beyond life casting, alginate works well for making moulds of objects — toys, natural forms, decorative objects, relief panels — where you want a quick, precise mould without the longer process involved in latex mould making.
Plaster of Paris: the classic casting material
Plaster of Paris is calcium sulphate hemihydrate — a fine white powder that, when mixed with water, sets to a hard, solid white material. It has been used in art, architecture, and craft for centuries and remains the most widely used casting material for home and studio use.
We stock Moulding Powder Plaster of Paris at £9.25 at Craft and Canvas.
How to mix plaster of Paris
The most important rule in mixing plaster is to add plaster to water, never water to plaster. Adding the powder to water gradually — sprinkling it in and allowing it to sink — produces a lump-free, even mix. Adding water to plaster causes lumps that are very difficult to remove.
The standard ratio for casting is approximately two parts plaster to one part water by volume. Sieve or sprinkle the plaster powder into the water and allow it to soak undisturbed for two minutes before stirring. Stir gently and thoroughly for two to four minutes until the mix reaches a smooth, creamy consistency with no lumps. Avoid vigorous stirring which introduces air bubbles into the mix — air bubbles produce holes and imperfections in the finished cast.
Pour the mixed plaster carefully into your mould, filling it halfway first and gently rotating the mould to release any trapped air before filling completely. Tap the sides of the mould lightly to encourage air bubbles to the surface.
Plaster sets through an exothermic reaction — it gives off heat as it hardens. When the mould feels warm to the touch the plaster is setting. Setting time is typically 10 to 15 minutes for the Moulding Powder we stock. Leave the cast undisturbed until the warmth has fully dissipated and the plaster has cooled to room temperature before demoulding — removing the cast from the mould too early risks damaging the still-soft plaster.
What you can do with plaster
Plaster of Paris is used as the casting material in alginate life casting — filled into the alginate mould to produce a hard positive reproduction. It is also used directly in Modroc construction, for casting into latex moulds, and for making simple press moulds from clay or sand. The dried plaster can be sanded, sealed, and painted with acrylic paint to produce a finished decorative piece.
Liquid latex: flexible moulds for repeat casting
Liquid latex is a rubber solution that sets by evaporation of its water content, building up into a flexible, elastic rubber skin when applied in layers. Unlike alginate — which sets in minutes and is single-use — a latex mould takes longer to make but can be used repeatedly to produce multiple casts, making it the right choice whenever you want to reproduce an object more than once.
We stock Gedeo Liquid Latex at £10.50 and Seawhite Liquid Latex 1 litre at £27.99.
How liquid latex mould making works
Liquid latex is applied to the surface of the object being moulded in layers, allowing each layer to dry completely before applying the next. The latex is painted on with a brush — the first coat is the most critical as it captures all the fine surface detail. Subsequent coats build up the thickness and strength of the mould.
A minimum of six to eight coats is typically needed for a usable mould, with more coats for larger or more complex objects. Each coat is touch-dry in approximately 20 to 30 minutes in normal conditions, meaning a complete mould can be built up in a single day of working in stages. The finished latex skin is then peeled carefully from the object, inside out if necessary for objects with undercuts, to produce a flexible rubber negative.
Once made, a latex mould can be filled with plaster, wax, or other castable materials repeatedly to produce multiple identical casts. Between uses, store the mould in a cool, dry place away from direct sunlight, which degrades latex over time.
When to choose latex over alginate
The choice between alginate and latex comes down to how many casts you need. For a single cast — particularly a life cast where the original is a living person — alginate is faster, simpler, and perfectly suited. For any situation where you want to reproduce an object multiple times, latex mould making is the correct approach. The additional time invested in building up the latex mould pays back immediately if you want to produce more than one or two casts.
Liquid latex is also skin-safe for most people, but some individuals have latex sensitivity or allergy. Anyone who knows they have a latex allergy should not use liquid latex on skin. Both latex products we stock are suitable for making moulds of objects and craft applications without skin contact.
Modroc: quick construction and support shells
Seawhite Modroc Roll at £1.95 is plaster-impregnated bandage — fabric strips pre-loaded with plaster that set hard when dipped in water. It is not a mould making material in itself but is closely related to the mould making process in two ways.
First, Modroc is used as a support shell or mother mould over a latex mould — the flexible latex mould alone can be too floppy to hold its shape when filled with casting material, so a rigid plaster bandage shell is made over it to provide structural support during casting.
Second, Modroc is used in its own right for quick three-dimensional construction — armatures, papier mâché-style structures, props, and sculptural work where a rigid surface is needed quickly. Applied over a balloon, a wire armature, or a found object, Modroc sets in 10 to 15 minutes and produces a light, strong shell that can be painted and finished in any way you choose.
At £1.95 per roll — 15cm wide and 275cm long — it is exceptional value and one of the most useful and versatile materials in the whole making range.
Putting it all together: a simple first project
The most natural first mould making project using the materials we stock is an alginate cast of a hand.
You will need: Alginate 500g, Moulding Powder Plaster of Paris, a bucket or deep container large enough to immerse a hand, a mixing bowl, water, and a stirring implement.
Mix the alginate with water in your container — enough to submerge the hand to the wrist. The mix should be pourable. Work quickly once the alginate is mixed. Immerse the hand in the alginate, holding it still in a relaxed, slightly spread position. Hold steady for three to four minutes while the alginate sets around the hand. The gel will change appearance as it sets — when it feels firm and rubbery it is ready. Carefully withdraw the hand by gently wiggling the fingers free, taking care not to tear the mould. You now have a detailed alginate negative of the hand.
Mix plaster of Paris to a smooth, creamy consistency and pour it carefully into the alginate mould, filling it in stages and tapping to release air bubbles. Allow to set completely — at least 20 to 30 minutes — before gently peeling the alginate away from the outside of the set plaster. The alginate tears away cleanly from the plaster surface, revealing the cast beneath.
Frequently asked questions about mould making
Is alginate safe for skin contact?
Yes — alginate is derived from seaweed and is completely non-toxic and skin-safe. It is the same material used in dentistry for dental impressions. It is safe for use on hands, feet, and other body parts.
How long does a plaster cast take to dry fully?
Plaster sets — hardens — within 10 to 30 minutes. But setting and drying are different things. A freshly set plaster cast contains significant water and is relatively soft. Full drying — the evaporation of this residual water — takes 24 to 48 hours depending on thickness. Wait until the cast is fully dry before painting or sealing it, as paint applied to damp plaster will not adhere properly.
Can I use plaster of Paris to make the mould as well as the cast?
Yes — plaster can be used to make rigid press moulds, particularly in combination with a clay original. Press a rolled slab of clay into a plaster mould and it absorbs moisture from the clay, allowing the clay to be removed intact. Plaster is also used as a support shell over latex moulds.
How many coats of liquid latex do I need?
A minimum of six to eight coats for a simple object. More complex objects or those with undercuts may need ten or more. The mould gets stronger and more dimensionally stable with each additional coat, so more is generally better up to a point. Allow each coat to dry fully before applying the next.
Can I cast into liquid latex moulds with materials other than plaster?
Yes — wax, resin, and various casting compounds can all be used with latex moulds. Plaster is the most accessible and affordable casting material for most beginners. Check compatibility with the specific casting material before using it.
Does the weather affect mould making?
Temperature and humidity both affect working times. Alginate sets faster in warm conditions and slower in cool ones. Plaster sets faster in warm conditions too. Latex dries faster in warm, dry conditions and more slowly in cool or humid environments. Working in a consistent room-temperature environment gives the most predictable results.
Shop mould making supplies at Craft and Canvas
We stock Alginate 500g at £19.99, Moulding Powder Plaster of Paris at £9.25, Gedeo Liquid Latex at £10.50, Seawhite Liquid Latex 1 litre at £27.99, and Seawhite Modroc Roll at £1.95 at Craft and Canvas in Hebden Bridge and online at craftandcanvas.co.uk.
Craft and Canvas | 3 Carlton Street, Hebden Bridge, HX7 8ER | craftandcanvas.co.uk